....The directions say from the electric fan sending unit (my Flex-a-lite) to run one wire to the black ground trigger with the other wire from the sending unit to ground. The problem is there is no ground on my sending unit. The sending unit has a metal probe that goes into the radiator, one wire to 12v switched and one to the positive wire on the fan.
Wire it as the directions say. Your existing mechanical adjustable thermostat switch will act as the temp switch, you just need to ground the other terminal on the thermostat switch.
Hope this helps...